Abstract：Molten salts play a key role in thermal energy storage of solar thermal utilization. The higher the upper limit of the usable temperature of molten salt, the greater the heat storage density and the higher the energy transfer efficiency. This work uses both experiments and quantum chemical calculations to study factors affecting the stability of two molten salts of Solar Salt and HITEC at a high temperature. Solar Salt and HITEC were firstly prepared, which involved drying of NaNO3 and KNO3 powder in a vacuum oven at 120℃ for 24 hours; mixing the dried NaNO3 and KNO3 in a mortar with 60%:40% (mass ratio) followed by grinding of the mixture; heating the ground powder mixture in a resistance furnace at 400℃ for 12 hours; cooling the molten salt down and grounding the sample to a powder form. The resulting material gave the Solar Salt. The HITEC were prepared by using the same method with mass ratio of KNO3, NaNO3 and NaNO2 being 53%:7%:40%(weight fraction). High-temperature stabilities of samples were studied by DSC-TG analyses. The decomposition of the products was analyzed by XPS. The nitrate decomposition into nitrite was simulated by a software based on B3LYP functional with a base group of 6-31+G*. Finally factors affecting the stability of the nitrate salts were discussed from the reaction mechanisms. The results showed that, when heated to 600℃, Solar Salt and HITEC mass losses were 2% and 1%, respectively. No metal oxides were produced or the production rate is too low to be observed. The difference in the metal ion proportion and the difference in the acid radical proportion were regarded as possible factors for the difference in the stabilities of the molten salts. The radii of the metal ions were found to be different, leading to different energies of intermediates and transition states. The movement of chemical equilibrium, caused by reactant, was considered as an additional reason for the different thermal stabilities of the Solar Salt and HITEC.